In general, a heat pump apparatus to be used for a cooling and heating apparatus and a hot water supply device uses air as a heat source.
Further, in districts where an outside air temperature is low, a heat pump apparatus using underground heat in heating has also come to be used.
In an air heat-source heat pump apparatus using heat of air as a heat source, when an outside air temperature is low in a heating operation, a heating capacity may be reduced due to reduction in suction pressure, frosting, or the like. Thus, operation efficiency of the heat pump apparatus depends on the outside air temperature.
In an underground-thermal-energy heat pump apparatus using underground heat, when an underground temperature is higher than an outside air temperature, the amount of collected heat can be increased, and hence the operation efficiency is higher than that of the air heat-source heat pump apparatus. However, when the underground temperature is lower than the outside air temperature, conversely, the operation efficiency of the underground-thermal-energy heat pump apparatus is worse than that of the air heat-source heat pump apparatus.
Further, although the underground temperature less varies throughout the year as compared to the outside air temperature, the variation width of the underground temperature depends on the district, depth, and season. Also for this reason, the operation efficiency of the underground-thermal-energy heat pump apparatus may be worse than that of the air heat-source heat pump apparatus.
As a measure for solving those problems, in Patent Literature 1, there is disclosed a technology of switching between an air heat-source heat exchanger, which is installed on the ground to use outside air as a heat source, and an underground heat-source heat exchanger, which uses underground heat collected by an underground heat exchanger buried in the ground as a heat source. In Patent Literature 1, a passage is switched so that the air heat source exchanger may be used when an outside air temperature is equal to or higher than a predetermined value or when a refrigerant temperature is equal to or higher than a predetermined value (for example, equal to or higher than a temperature at which the air heat source exchanger is frosted) and that the underground heat-source heat exchanger may be used when the refrigerant temperature is equal to or lower than the predetermined value.